There has been known a spark plug in which a ground electrode is welded to an end surface of a metallic shell, and a gap is formed between the inner circumferential surface of the metallic shell and a forward end portion of an insulator which holds the center electrode (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2003-223968; Japanese Patent Application Laid-Open (kokai) No. 2011-175985; International Patent Publication No. 2009/020141). Japanese Patent Application Laid-Open (kokai) No. 2003-223968; Japanese Patent Application Laid-Open (kokai) No. 2011-175985; and International Patent Publication No. 2009/020141 disclose that after formation of an end surface and an inner circumferential surface on a metallic shell through shaping, a ground electrode is welded to the end surface of the metallic shell. Japanese Patent Application Laid-Open (kokai) No. 2003-223968 further discloses that after the ground electrode has been welded to the metallic shell, an overflow (hereinafter called a “welding sag”) resulting from having overflowed onto the surface of the metallic shell is removed.
The techniques of Japanese Patent Application Laid-Open (kokai) No. 2003-223968; Japanese Patent Application Laid-Open (kokai) No. 2011-175985; and International Patent Publication No. 2009/020141 have a problem in that, in the case where the thickness of the metallic shell at an end surface thereof is relatively small as compared with the thickness of the ground electrode, the ground electrode is likely to deviate and drop from the end surface of the metallic shell when the ground electrode is welded to the end surface of the metallic shell. Also, techniques of Japanese Patent Application Laid-Open (kokai) No. 2003-223968; Japanese Patent Application Laid-Open (kokai) No. 2011-175985; and International Patent Publication No. 2009/020141 have a problem in that the inner circumferential surface of the metallic shell may deform due to the influence of heat generated when the ground electrode is welded to the end surface of the metallic shell. These problems become significant when the size of the spark plug is reduced.
The present invention addressed the above-mentioned problems, and can be realized as the following modes.